title: Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes.
abstract: OBJECTIVE
  The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes.
  
  
  METHODS
  Fourteen highly trained male triathletes (VO2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period.
  
  
  RESULTS
  Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group.
  
  
  CONCLUSIONS
  In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise.
authors.affiliation: 1Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense, DENMARK; 2Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Östersund, SWEDEN; and 3The Åstrand Laboratory, Swedish School of Sport and Health Sciences, Stockholm, SWEDEN.
authors.forename: Kasper Degn
authors.initials: KD
authors.lastname: Gejl
authors.forename: Lars Grøndahl
authors.initials: LG
authors.lastname: Hvid
authors.forename: Ulrik
authors.initials: U
authors.lastname: Frandsen
authors.forename: Kurt
authors.initials: K
authors.lastname: Jensen
authors.forename: Kent
authors.initials: K
authors.lastname: Sahlin
authors.forename: Niels
authors.initials: N
authors.lastname: Ørtenblad
mesh_terms: D000328:Adult; D056352:Athletes; D001642:Bicycling / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D003718:Denmark; D006003:Glycogen / Q000378:metabolism*; D006801:Humans; D017136:Ion Transport / Q000187:drug effects*; D019344:Lactic Acid / Q000097:blood; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D010809:Physical Fitness / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D019344:Lactic Acid; D006003:Glycogen; D002118:Calcium